2-stroke crankcase-charged internal combustion engine lubrication



April 20, 1965 W. D. SIMS ETAL 2-STROKE CRANKGASE-CHARGED INTERNAL COMBUSTION ENGINE LUBRIGA'I'ION Filed March 23, 1964 INVENTORS:

WILLIAM D. SIMS STANLEY S. SOREM THEIR ATTOR EY 3.,l79fi93 Patented Apr. 20, 1965 e if 3,179,093 Z-STROKE CRANKCASE-QGED INTERNAL COMBUdTlUN ENGINE LUBRKCATHGN William 1). Sims, Lafayette, and Stanley S. Sorem, Orinda, Calitl, assignors to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed Mar. 23, 1964, Ser. No. 353,836 3 Claims. (ill. 123-43) This invention relates to an improvement for two-stroke cycle crankcase-charged internal combustion engines and more particularly relates to a new piston ring arrangement by which lubrication of the upper cylinder walls, the degree of sparkplug fouling and the degree of combustion chamber deposition is improved.

As two-stroke crankcase-charged gasoline engines have increased in specific output, and particularly outboard boat engines and lightweight multi-purpose engines employed for such uses as chain saws, the lubrication requirements have become much more stringent. Although lubrication in this type of engine for the heretofore required outputs appears to be satisfactory, as the specific output continues to increase areas of the engine which are presently sufticiently lubricated present increased lubrication problems. Such an area of the engine is the upper cylinder walls, i.e., the piston ring belt.

In a two-stroke crankcase-charged engine, lubricating oil is supplied as a mixture with the fuel. This fuel-oil mixture is aspirated into the crankcase with the entering combustion air by upward piston motion. Until recently, the most frequently advanced theory for engine lubrication in a two-stroke cycle crankcase-charged engine was the mist theory. According to this theory, the oil-fuelair mixture, which is in the form of a mist as it enters the crankcase, dispersed itself for lubrication of the salient bearing surfaces throughout the engine. Recent tests utilizing high speed photography, however, have shown this theory to be largely a myth. According to the aforementioned tests, upon striking the inlet valve and other surfaces of the engine, the oil droplets in the fuel-oil-air mixture coalesce forming visible droplets which then flow along the crankcase surfaces under the influence of adjacent air streams and gravity and accumulate in the bottom of the crankcase. The oil in the bottom of the crankcase is agitated by the rotation of the crankshaft and the connecting rod dipped into the pool of oil, and hence becomes substantially a splash lubricating system.

Although such a splash lubrication system provides adequate lubrication for the bearings and for the lower cylinder walls, the upper cylinder walls, however, are too remote to profit directly from splashed oil. Consequently, the upper cylinder walls, if they are to be lubricated at all, must be lubricated by oil drops still entrained by the fuel-air mixture as it is forced from the crankcase and into the combustion chamber by downward piston displacement or alternatively by oil carried up along the cylinder walls by the action of the piston rings. This alternate path, however, is not likely to supply much oil to the upper cylinder walls in a conventional two-stroke engine equipped with the usual fiat faced piston rings, i.e., rings having a rectangular cross section. This is due to thefact that the net oil flow past piston rings of nor mal (rectangular cross section) shape, when oil is supplied to both sides of the ring, is in the direction of the pressure gradient. Since in an operating engine oil is supplied to both sides of the ring and the pressure gradient is downward or toward the crankcase, the net oil flow past the rings is also toward the crankcase of the engine, resulting in the total lubrication of the upper cylinder walls being from oil droplets carried by the fuel-air mixture into the combustion chamber.

Oil reaching the upper cylinder walls by way of the combustion chamber, however, has suiferedsome degradation from thermal oxidative conditions in the chamber and furthermore tends to be contaminated by available products of complete and partial combustion. This oil, therefore, produces less of a lubricating effect on the upper cylinder walls then could be derived with the oil contained in the intake mixture. Furthermore, once the contaminated oil is on the cylinder walls, the oil is subjected to the transport etiect of the rings and pressure gradients and is moved toward the crankcase where it is mixed with the oil therein. Some of the oil dragged back into the crankcase will then recycle to the combustion chamber as droplets in the fuel-air mixture and be returned to the crankcase for possible further recycling. Although such recycled oil is diluted with the new oil being continuously supplied to the crankcase, it is obvious that the degree of purity of the oil being returned to the combustion chamber is less than that of the incoming new oil and hence tends to further decrease the lubricating effect of the oil reaching the upper cylinder walls.

In addition to the problem of improving lubrication for the upper cylinder walls, oil droplets entering the combustion chamber via the inlet ports are easily carried by the incoming air to directly strike the sparkplug or other surfaces of the combustion chamber. Since the residue left by these impinging oil droplets is a function of the solid content of the oil, such impingement by recirculated oil into which the residue from combustion of tetra ethyllead, known to be lead oxyhalide, oxysulfates and oxides, has been milled by piston and ring motion is considerably greater than the solid content of oil which has made only a single pass into the combustion chamber, i.e., arriving or new oil. This dropwise impingement of oil containing such electrically conductive solids on the sparltplugs and on the combustion chamber walls themselves results in short circuiting deposits forming on the sparkplugs with eventual fouling or misfiring thereof, and harmful deposits forming on the combustion chamber walls. Both of these conditions decrease the overall efiiciency of the engine. Thus, it can easily be seen that minimization of the recycled oil moving along the cylinder walls toward the crankcase will produce reductions in sparkplug and other combustion chamber deposits as well improvements in the cleanliness of the upper cylinder walls, resulting in more efiicient engine operation.

Accordingly, it is an object of this invention to provide for the improved operation of a two-stroke cycle crankcase-charged internal combustion engine by the reduction of the quantity of oil being recycled in the engine. It is another object of this invention to provide for the increased and more effective lubrication of the upper cylinder wallof a two-stroke cycle crankcasecharged internal combustion engine. 7

Briefly, according to the invention, the above objects are attained by replacing one or more of the conventional flat-faced piston rings used in two-cycle engines with piston rings which scrape oil in only one direction and are positioned such that they scrape the oil on the cylinder Walls toward the combustion chamber and away from the crankcase. Such piston rings mounted on the piston in this fashion, will scrape the uncontaminated oil splashed onto the lower Walls of the cylinder toward the combustion chamber and thereby lubricate the upper cylinder walls with the uncontaminated oil. Furthermore, such rings will produce a net flow of oil toward the combustion chamber and thereby minimize the travel of contaminated oil from the combustion chamber to the crankcase.

The above objects and advantages of the invention will be more clearly understood from the following description when taken in conjunction with the accompanying drawings, wherein:

- FIGURE 1 is a vertical sectional view of a two-stroke cycle crankcase-charged internal combustion engine showing a piston having rings mounted thereon according to the invention; and,

FIGURE 2 is a fragmentary vertical section of a piston showing a modification of the invention.

Referring now to FIGURE 1, there is shown a typical two-stroke cycle crankcase-charged internal combustion engine having a crankcase l and a cylinder 2 which, as illustrated, is of the air-cooled type and hence is provided with projecting cooling fins 3. Arranged to reciprocate in the cylinder 2 is a conventional piston 41 with a solid periphery and having a connecting rod 5 associated therewith which is connected via a crank 6 to a crankshaft 7 in the usual manner. The piston 4 is provided with piston rings -8-1 which are seated in circumferential piston ring grooves 11 adjacent the head 12 of the piston. The cylinder 2 is also provided with an exhaust port 13 formed in the wall thereof; an inlet port 14 formed in the wall of the cylinder which is in communication via a channel 15 with the interior of the crankcase 1; and a sparkplug 17 which is mounted in the integrally formed head portion 18 of the cylinder and which at the proper time Will ignite the fuelair mixture in'the combustion chamber 19 formed between the head portion of the cylinder and the head 12 of the piston.

In the operation of a crankcase-charged two-stroke cycle internal combustion engine, the fuel-air mixture supplied to the engine is aspirated into the crankcase via the crankcase port 26 during the upward or compression stroke of the piston 4-. After combustion of the fuel in the combustion chamber 19, as the piston moves toward the crankcase, the inlet and exhaust ports 14 and 13 become uncovered and the fuel-air mixture in the crankcase 2 is forced by the downward movement of the piston into the cylinder via the'channel 15 and the inlet port 14. The fuelair mixture. being forced into the cylinder will scavenge the exhaust from the cylinder in the manner usual with engines of this type and force the exhaust out of the ex haust port 13. As the piston moves back toward the head of the cylinder on the next compression stroke, the ports 13 and 14 are then closed and the fuel-air mixture trapped in the cylinder is compressed within the combustion chamber 19 where it is subsequently ignited by the sparkplug 1'7 and the same cycle again takes place.

As pointed out above, the oil for lubricating this type of engine is supplied with the fuel-air mixture being intro duced into the crankcase 1. Upon striking the inlet valve (not shown) and other surfaces, the oil droplets coalesce and fiow along the walls of the crankcase surfaces and finally settle in the lowermost portion of the crankcase forming a pool of oil therein. Lubrication of the bearings is then accomplished largely by the splashing caused by the connecting rod 5 and crank 6 dipping into the pool of oil as they revolve during the piston strokes. Lubrication of the lower cylinder walls is also accomplished largely by the splashes of oil coming from the revolving crank and connecting rod. The upper cylinder walls, however, due to their remote location from the rotating parts and because of the piston 4 are not directly lubricated by splashed oil. Consequently, the upper cylinder walls must be lubricated either by oil droplets entrained in the fuel-air mixture as it is forced from the crankcase 1 into the combustion chamber 19 by the downward move ment of the piston or must be carried along the cylinder walls by the action of the piston rings 8-16. Neither of these methods, however, provide adequate lubrication for the upper cylinder walls. The former because the oil reaching the upper cylinder walls has been contaminated by the products of combustion and the latter because the conventional flat-faced piston rings utilized in crankcasecharged two-stroke cycle internal combustion engines tend to produce a net flow of oil in the direction of the pressure d gradient, which is toward the crankcase 1. As further pointed out above, this scraping of the oil on the upper cylinder walls toward the crankcase tends to further contaminate the pool of oil lying in the crankcase. Since some of the oil drop-lets entrained in the fuel-air mixture forced into the combustion chamber results from drops of oil splashed from the rotating connecting rod, the contamination of the pool of oil in the bottom of the crankcase l. causes oil of lower quality to reach the upper cylinder walls and an increased quantity of combustion prodnets to reach the combustion chamber.

in order to increase the quantity and the quality of the oil reaching the up er cylinder walls, according to the invention, the conventional square-faced compression type piston rings are replaced with piston rings which scrape oil in only one direction. Such rings are old and well known in the art as oil control rings for four-stroke cycle engines are referred to, according to their specific structure, as taper faced, bevel faced, bevel scraper, etc. Contrary to the usual manner of inserting these rings into the grooves, i.e., to scrape oil toward the crankcase, the rings are placed in the grooves in an inverted manner in order to scrape the oil on the cylinder walls toward the combustion chamber and away from the crank-case. Since such rings have the ability of move oil against the pressure gradient, an eifective pumping of oil toward the combustion chamber results. 7

As shown in the drawings, the piston 4 is preferably provided with a taper faced type piston ring 3 and a pair bevel scraper type piston rings 9 and 1d. The rings S-3itl are mounted in the grooves 11 such that the tapered or beveled periphery of the piston rings slopes inwardly, i.e., the outer circumference of the piston rings decreases, in a direction toward the crankcase. With the piston rings mounted in'this manner, on the downward stroke of the piston 4, the tapered or beveled periphery of the piston rings 8-19 tends to cause the rings to skim over any oil on the cylinder walls. On the upward stroke, however, the oil is scraped along the cylinder walls toward the combustion chamber 19 by the upper edges of the piston rings 8-16 Once in the combustion chamber, the oil is forced out of the exhaust port 13 on the next power stroke of the piston. This results in preventing the contaminated oil on the upper cylinder walls from reachins the crankcase for recycle and furthermore, since a relatively large quantity of uncontaminated or high quality oil is on the lower portions of the cylinder wall due to splashing, the piston rings S-ld will, on the upward stroke of the piston, push or pump this oil to the upper cylinder walls resulting in increased and more effective lubrication of this portion of the engine.

While two different types of piston rings havebeen shown, i.e., taper faced and bevel scraper, it is understood that any ring which moves oil in one direction may be used, and that the rings may be all of the same type. Furthermore, while it is recognized that rings such as bevel scraper rings which have a recess 21 in the scraping edge are more efiicient in controlling the direction of oil flow, preferably the ring closest to the head of the piston is of a type not containing the recess. This is done both for increased engine compression and to eliminate the possibility of the recess 21 filling with combustion by-prodnets and thereby destroying the effect of the ring.

Although the above-described modification of the conventional crankcase-charged two-stroke cycle internal combustion engine results in greatly improved operating conditions for the engine, as shown in FIGURE 2, still further increased lubrication can be achieved by providing a piston ring groove 2-3 on the skirt portion of the piston 4 having mounted therein an additional single direction oil scraping piston ring 24 which is also positioned to scrape oil toward the combustion chamber 19. Since as pointed out above, most of the oil on the lower cylinder walls results from the splashing of oil from the rotating connecting rod 5, due to the tangential direction of this splashed oil, the density or quantity of high quality oil existing on the cylinder wall increases in a direction toward the crankcase. Consequently, the piston ring 24, because of the position on the skirt of the piston 4, reaches a lower portion of the cylinder Wall and hence pumps or scrapes a greater quantity of uncontaminated oil toward the combustion chamber than is possible with the piston rings mounted near the head portion of the piston.

Obviously various modifications of this invention are possible in light of the above teachings without departing from the spirit of the invention. understood that Within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

We claim as our invention:

1. In a two-stroke cycle crankcase-charged internal combustion engine wherein the lubricating oil is supplied to the engine in a mixture with the fuel, and wherein said engine has a cylinder, a piston, having a head and skirt portion, mounted for movement within said cylinder, a plurality of circumferential piston ring grooves situated near the head portion of said piston, and a piston ring mounted in each of said grooves; the improvement comprising: at least one of said piston rings being of the type It is therefore to be a for scraping oil in only one direction and positioned in its respective groove to scrape the oil on the walls of said cylinder toward the combustion chamber associated with said cylinder and against the pressure gradient in said cylinder to produce a net flow of oil on the cylinder Wall toward the combustion chamber.

2. The combination of claim 1 wherein all of said piston rings are of the type for scraping oil in one direction and are positioned to scrape the oil on the walls of the cylinder toward the combustion chamber.

3. The combination of claim 1 including an additional piston ring groove mounted on the skirt portion of said piston; and, a piston ring of the type for scraping oil in only one direction mounted in said last-mentioned piston ring groove and positioned to scrape the oil on the cylinder Walls toward the combustion chamber associated with said cylinder.

Reterences Cited by the Examiner UNITED STATES PATENTS 955,151 4/10 Gilliard 123-73 1,342,273 6/20 Blac-he 92-453 FRED E. ENGELTHALER, Primary Examiner. 

1. IN A TWO-STROKE CYCLE CRANKCASE-CHARGED INTERNAL COMBUSTION ENGINE WHEREIN THE LUBRICATING OIL IS SUPPLIED TO THE ENGINE IN A MIXTURE WITH THE FUEL, AND WHEREIN SAID ENGINE HAS A CYLINDER, A PISTON, HAVING A HEAD AND SKIRT PORTION, MOUNTED FOR MOVEMENT WITHIN SAID CYLINDER, A PLURALITY OF CIRCUMFERENTIAL PISTON RING GROOVES SITUATED NEAR THE HEAD PORTION OF SAID PISTON, AND A PISTON RING MOUNTED IN EACH OF SAID GROOVES; THE IMPROVEMENT COMPRISING: AT LEAST ONE OF SAID PISTON RINGS BEING OF THE TYPE FOR SCRAPING OIL IN ONLY ONE DIRECTION AND POSITIONED IN ITS RESPECTIVE GROOVE TO SCRAPE THE OIL ON THE WALLS OF SAID CYLINDER TOWARD THE COMBUSTION CHAMBER ASSOCIATED WITH SAID CYLINDER AND AGAINST THE PRESSURE GRADIENT IN SAID CYLINDER TO PRODUCE A NET FLOW OF OIL ON THE CYLINDER WALL TOWARD THE COMBUSTION CHAMBER. 